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The article does not even discuss the obvious alternative that is perfectly consistent with relativity: all events in your past light cone "exist" (a better ter

• panda-giddiness 4y ago
  The author hasn't neglected to consider light cones – she literally describes them in her post, as evinced by her mentioning 45 deg lines on a spacetime diagram, causally connected/disconnected events, etc. Of course you have described a perfectly good way of thinking about the past/future (and to your credit, she surely should've included what you described). But in her defense, what you have described is also the most obvious explanation consistent with special relativity – the "past" and "future" outside one's line cone cannot be objectively defined, as you could always (hypothetically) boost to another frame in which the disconnected past becomes the disconnected future (and vice versa). Undoubtedly the author is aware of this fact, so I assume the purpose of this blog post was simply to motivate other hypotheses about the nature of time.
  • pdonis 4y ago
    > The author hasn't neglected to consider light cones She describes them, but does not consider their implications for the structure of spacetime. > you could always (hypothetically) boost to another frame in which the disconnected past becomes the disconnected future (and vice versa). No, you can't. That's the whole point of looking at light cones instead of spacelike "now" slice. Light cones are invariant; they're the same in all frames. Spacelike "now" slices are not. Also, you can't do a Lorentz boost that turns "future" into "past" or vice versa.
    • jamesmaniscalco 4y ago
      I think OP means “disconnected past” as the spacelike-separated region south of the “now” slice. Boosting could transform points in this region into the spacelike-like separated region in the “future” (i.e. the “disconnected future), no? The points are not transforming across the light cone boundary, just across the arbitrary “now” plane.
      • panda-giddiness 4y ago
        Yes, this is what I meant. By "disconnected" I meant causally disconnected, i.e. spacelike separated. Sorry for not making that clearer! And yes, you can always boost to a frame such that the "order" of spacelike separated events will change (i.e., reverse which event is above and which is below a line of simultaneity). Since you can't objectively say which event happened first, they must be unable to influence each other, hence "causally disconnected".
• scrollbar 4y ago
  This sounds very interesting, thank you for sharing. Could you recommend any resources to learn about these three regions, ideally for someone without formal physics training?
  • smallerfish 4y ago
    Carlos Rovelli - any of his recent books.
    • goldenkey 4y ago
      Particularly his work on relational quantum mechanics and course-graining model of entropy. https://en.wikipedia.org/wiki/Relational_quantum_mechanics https://en.wikipedia.org/wiki/Relational_quantum_mechanics https://arxiv.org/abs/1407.3384 https://arxiv.org/abs/1407.3384
  • pdonis 4y ago
    Penrose's The Emperor's New Mind contains a good layman's discussion of it. (It also contains a good example of a layman's work that makes the same mistake the article under discussion here does: even after going to all the trouble to explain in detail how the three regions I described work, and how "now" slicings, unlike the three regions I described, are frame-dependent and don't have physical meaning, he then makes an invalid argument for the "block universe" viewpoint that depends on ignoring the three regions and treating "now" slicings as though they do have physical meaning. It's an excellent example of how even experts of the stature of Penrose can get things wrong.)
• codeflo 4y ago
  That’s my favorite way to think about this as well, but it’s very geometric and maybe not so intuitive to explain in a video that targets a non-mathematical audience. That physicists aren’t aware of this interpretation might be a bit of a stretch, that’s at least not been my experience.
• Schiphol 4y ago
  The problem with that is that, then, what exists (in the unrestricted sense of 'exist' the OP is after) becomes event-relative: not everything that exists from your vantage point exists from the vantage point of someone in Alfa Centauri or whatever. You may want to live with this 'metaphysical relativism', but it is hardly better than the solution offered by the OP, which is the standard one.
  • vehemenz 4y ago
    Or we can reject ontological anti-realism. That is, what "exists" in the first place are linguistic conventions constrained by physics, our understanding, and practicality.
    • vehemenz 4y ago
      Please note the inverted commas around 'exists' in my comment. They are there for a reason. I can imagine the layman ascribing some mystical quality to ontological anti-realism, but, ironically, the view acts to dispel the mystical quality of uncritical "existence" claims that many take for granted (much like theism, moral realism, etc.).
  • pdonis 4y ago
    > what exists (in the unrestricted sense of 'exist' the OP is after) becomes event-relative Well, of course; that's how we already use the term "exist" in ordinary language. We say that our past of today "exists", but tomorrow does not, at least not yet. But tomorrow, we'll say that what we now call tomorrow does "exist". So our ordinary language concept of "exists" already takes into account that what "exists" changes over time.
    • Schiphol 4y ago
      The commonsense tensed 'exist' is not a good model to 'only my light cone exists'. The former says that events in my causal past and future do not exist, for example. The latter only makes sense as a proposal about some unrestricted notion of existence. More or less: what is the smallest catalogue of entities I need to commit myself to, if I want to make sense of the science?
• sandgiant 4y ago
  I don't think physicist really misinterpret this if you asked them. The problem is, as you say, that the "past" and "future" only have a physical meaning inside your own light cone. As alluded to in the video, you cannot compare space and time differences without defining a common reference frame, so it doesn't really make sense to talk about a past and future in spacelike separated regions of spacetime. Special relativity is great because it produces some very non-intuitive conclusions about our Universe, and it's always lovely to explore new mind-bending questions arising from these conclusions, especially in outreach pieces like this. I think conceptual discussions like these can be very enlightening, but its not something that keep a lot of physicists up at night.
• nathan_compton 4y ago
  In General Relativity events in your past light cone can be events in your future light cone. In Loop Quantum Gravity, the idea of a past and future lightcone become discrete and entirely local to a specific element of spacetime. Consequently, assigning elements of a past light cone to "reality" becomes ambiguous: _which_ past light cone? At the level of LQG all you can describe is relations between points of spacetime and the idea of past/future becomes ambiguous. Another modern approach to quantum gravity (energetic causal set theory) does re-impose a sort of global ordering (in the sense reality is a series of states updated with a single rule depending on the momenta of previous realizations of the state elements) but in that theory the past really is gone. This is similar to the (more classical) approach of Shape Dynamics (a general relativity-like theory of gravity) where all that relates a "current" snapshot of time to a "previous" snapshot is that it minimizes a particular mathematical quantity. I am definitely not an expert in LQG or GR or Shape Dynamics, but the story isn't quite so obvious and simple as you make it out to be and that may be why you are unsatisfied with how physicists talk about it.
  • pdonis 4y ago
    > In General Relativity events in your past light cone can be events in your future light cone. Only in spacetimes with closed timelike curves, such as the Godel Universe. Those spacetimes are considered by physicists to not be physically realistic. All physically realistic spacetimes, including all of the ones we use to actually model things in the actual universe, are globally hyperbolic, and in a globally hyperbolic spacetime the same event cannot be in both the past and the future light cone of some other event.
    • nathan_compton 4y ago
      They appear in the Kerr metric, which isn't particularly exotic except for the fact that realistic spinning black holes clearly don't have all the symmetries of a Kerr black hole. I would not be surprised to learn that the question of CTCs in realistic spinning black hole solutions is open, but I am not an expert in General Relativity. We certainly use the Kerr black hole solution to model things in the actual universe. At the very least, the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood (as do the other more outre theoretical avenues I pointed out). I think its pointless to be glib about the reality of the past, given that fact.
• deleted 4y ago
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• raattgift 4y ago
  I don't understand your objection to the slicing procedure. The central assumption is that for a reasonably physical spacetime there exists a unique maximal globally hyperbolic solution to the field equations, justifying the use of the initial value formulation of General Relativity. [aside: [1]] Once we do that we can take any arbitrary spacelike hypersurface Σ and in ADM-ish vocabulary observe the compact closure conditions (Σ has a spacelike geodesic from every p in Σ to every other p in Σ; every maximal spacelike geodesic has a nonempty open segment in Σ; every maximal timelike geodesic intersects Σ), the "thin" condition (no nomepty segment of a null geodesic is found in Σ), and a variety of causal conditions (e.g. no timelike geodesic connects points in Σ; no maximal timelike or null geodesic intersects Σ more than once). One can translate these features to your cones-only view. On a Minkowski diagram with p at the origin, Σ can have any shape or orientation compared to the light cones as long as p -- and nothing else in either of p's light cones -- appears on Σ. We can do the same for any other p in Σ, or we can decide whether the origin of any pair of light-cones is in Σ or not. We can also probably build Σ by hand from a principled choice of available light-cones. (One would be tempted to call that "initial data"). Is Σ physical? Of course not! But it can be used as the basis for physics that describes point-coincidences on other spacelike hypersurfaces, which we can then verify. > The future light cone is what can possibly be changed based on things we do here and now In which of your three regions do we find the freedom to do this? Is here-and-now not fully determined by the "fixed and certain" content of its past lightcone? Is the "we" part of "what we do here and now" describing a point on the manifold or is it describing a set of points on the manifold? Choose a point somewhere in the middle of "we" -- in which of the three regions are its other points? I struggle to answer that without thinking about an extended "we" with many entries in Σ (including photons (and other carriers of body forces) bouncing around within or even passing through the extended body). Maybe it's much easier for you. I wonder how you'd do if you were to think of grossly non-Eulerian (and even non-timelike) observers in a Big Crunch universe. In a Big Bang universe all points are in the future light cone at BB; in a BC universe all points are in the past light cone at BC. At every other point, only some points are in the two lightcones at that point. We get a BC from a time-inversion of our BB->approx de Sitter universe (e.g. for an observer with differently-aligned arrow-of-time). Assuming a long period of matter sparseness, in a BB->BC universe, we also get a ~BC universe if we cut away the expanding part; if we cut away the recollapsing part, we get something that's hard to distinguish from a BB->~dS universe like our own. In a BB->~dS universe I think we'd agree that the future light cones of two observers coincident at p can eventually fully separate. Can we agree that in a BC universe the past light cones of two observers at p can be at least somewhat, or even (practically) fully, non-overlapping (our observers need not be either "Eulerian" comovers with the metric contraction, and can even move lightlike, and there can be a cosmic deflation just prior to the big crunch)? In an ADM(-like) picture we can fairly straightforwardly use ancestor slices of Σ in the foliation and decide that a pair of photons will be at some point p on Σ. I also think it makes a slice through a spacetime-filling electromagnetic field easier to think about, ditching the idea of bare photons as independent objects. Again it doesn't matter about the timelike direction of the slicing, and one might want to recalculate slicing along some other choice of timelike axis to try to spot coordinate artifacts. However maybe you might object that if we don't have full data for the ancestor of Σ, we're somehow not better off than in the final sentence of your second paragraph. - -- [1] Weaker causality, such as stable causality in the sense that arbitrarily small metric perturbations don't give rise to CTCs, can still be used but at the cost of solution uniqueness, and weaker still at the cost of smearing out the Cauchy surface so that initial data is not all at t=0 but in some function defined over 0 >= t >= -V^{-1} where V is the scale of strong causality violation.